Correct Response: B. Circadian rhythms are periodic biological responses that occur on an approximately 24-hour cycle. Circadian rhythms are independent of external stimuli and would thus continue to occur under constant environmental conditions. Phototropism (A) is a directional response in which plants grow toward a light source. A day-neutral behavior (C) is independent of the relative amount of light and dark hours received by a plant. However, day-neutral behaviors do not occur on a 24-hour cycle. A photoperiodic response (D) depends on the ratio of light-to-dark hours a plant receives and would not occur under constant environmental conditions.
